The invention relates to a method of manufacturing optical fibres, in which glass is deposited in layers on the inner wall of a glass tube which is heated to a temperature between 1100.degree. and 1300.degree. C., and simultaneously on a glass rod which is arranged inside the glass tube, by leading a reactive gas mixture through the glass tube at a pressure between 1 and 30 hPa, while inside the glass tube a plasma is made to reciprocate strokewise between two reversal points after which the glass tube is made to collapse, following the deposition of a quantity of glass which corresponds to the intended construction of the optical fibre, so as to form a solid preform from which optical fibres are drawn.
A glass tube and a glass rod are to be understood to mean herein a substrate tube or a tube to be coated and a rod, which consist of synthetically manufactured amorphous silica or of amorphous silica manufactured from quartz crystals by melting fused silica, quartz glass, with the tube material possibly being doped, or which consist of both synthetically manufactured amorphous silica and amorphous silica manufactured from quartz crystals by melting (fused silica, quartz glass), with the tube material optionally being doped. The deposited glass consists of synthetically manufactured amorphous silica which is optionally doped.
The manufacture of optical fibres or optical waveguides according to the above-mentioned method is known from, U.S. Pat. No. Re. 30,635 and U.S. Pat. No. 4,314,833, the version in which the glass is simultaneously deposited on a glass rod arranged inside the glass tube, being known from U.S. Pat. No. Re. 30,635. The method of manufacturing except for said version is in practice referred to as "nonisothermal plasma-CVD-method" (nonisothermal PCVD method, in which P=plasma and CVD=chemical vapour deposition=reactive deposition from the gas phase). In this method, glass layers are directly deposited from the gas phase on the inner wall of the glass tube (hetereogeneous reaction). In this way, the formation of glass soot in the gas phase is prevented; this is described in greater detail in, in particular, U.S. Pat. No. 4,314,833.
By means of the PCVD method both graded-index fibres and stepped-index fibres can be manufactured, with quantities of glass being deposited which correspond to the relevant construction of the fibre.
Single-mode optical fibres having radially symmetrical refractive index profiles can transmit both orthogonally polarized HE.sub.11 -modes. Mode-coupling is obtained by quasi statistically distributed internal and external interferences, so that light transmission while preserving the original direction of polarization is impossible with these fibres. However, polarization-retaining optical fibres are of great interest for applications in interferometry, in coherent optical communication systems, in non-linear optics etc. It is known that such fibres can be obtained by using refractive index profiles which are not radially symmetrical, i.e. having elliptical or other unround fibre cores (for example EP Patent 47,037, U.S. Pat. No. 4,106,847) or by stress-induced birefringence in the core by means of a non-circular, stress-producing optical cladding (for example U.S. Pat. No. 4,274,854). Many appropriate structures for polarization retaining fibres and methods of manufacturing such structures are described by, inter alia, R. H. Stolen et al. (Electron. Lett. 18 (1982) 764-765), T. Hosaka (Electron. Lett. 17 (1981) 530-531, R. D. Birch et al. (Electron. Lett. 18 (1982) 1036-1037) and S. C. Rashleigh and R. H. Stolen (Fiberoptic Techn. (May 1983) 155-160). As regards internal coating methods, all known manufacturing methods are based on mechanical processes to which the substrate tube is subjected (for example grinding or polishing), on deformation during collapsing using an underpressure or on chemical etching (for example gas phase etching or etching in combination with photolithographic means). However, all these techniques have the disadvantage that interfering with the customary fibre-manufacturing process is necessary, causing the introduction of impurities or problems with cracks occurring in the preform, or that the desired geometrical and optical profiles are only reproducible with great difficulties.